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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power supply for a three-phase electric motor from a single-phase network with speed control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors Asynchronous electric motors (including three-phase ones) are widely used in everyday life and in production to drive machines and mechanisms, the speed of which is constant or variable using gearboxes with a variable gear ratio and other mechanical devices. Where it is necessary to smoothly adjust the shaft speed, preference is given, as a rule, to more expensive and less reliable collector electric motors, for which this operation is simple to perform - it is enough to change the supply voltage or current in the field winding. To control the shaft speed of an asynchronous motor, it is necessary to change not only the voltage, but also the frequency of the alternating current in its windings. The author of the proposed article talks about his solution to this problem. The device he developed makes it possible to feed an asynchronous three-phase motor with a power of up to 3,5 kW from a single-phase network and change its rotational speed by more than 10 times. Often there is a need to smoothly change the speed of machines and mechanisms equipped with an electric drive. Commonly used in such cases, collector electric motors are expensive, require periodic maintenance and are inferior to asynchronous ones in reliability, service life and weight and size indicators. The industry produces devices for frequency control of the speed of rotation of asynchronous motors. These devices are complex and expensive, so they are used only in critical cases, for example, in the drives of CNC machines. Schemes of such regulators for self-production were also published in the journal "Radio" [1, 2]. Unfortunately, they are designed for engines of very small power. The main problem that arises in the development of a frequency controller is the need to change, together with the frequency, the effective value of the voltage applied to the motor windings. The fact is that with a decrease in the frequency of the alternating current, the inductive resistance of the winding decreases, which leads to an unacceptable increase in the current flowing through it. To avoid overheating of the winding and saturation of the stator magnetic circuit, it is necessary to reduce the motor supply voltage. One way to do this, recommended in [3], is to connect the motor through an adjustable autotransformer, the moving contact of which is mechanically connected to the frequency controller. The method, it must be said, is very inconvenient, since the mass and dimensions of the autotransformer are comparable to those of the engine itself, and the reliability of the movable contact when transmitting high power is questionable. It is much more convenient to change the effective voltage value using pulse-width modulation (PWM) [4]. The proposed adjustable power supply for an asynchronous three-phase electric motor is based on just such a method. The source is built according to the scheme shown in Fig. one.
A powerful rectifier, which is part of the power supply and protection unit of the BPZ, converts a single-phase alternating voltage of 220 V 50 Hz into a constant 300 V. Using three dual power switches SK1 - SKZ, the windings of a three-phase electric motor M1 are switched, connecting them in the required order and polarity to the rectifier output . Circuits VD1L1 and VD2L2 protect the keys from load current surges. The pulses that control the keys are generated by the FIA block - the shaper of the control pulses. There are several more low-power rectifiers in the BPZ for powering the FIA and SC, as well as a current protection unit that disconnects the device from the network when the permissible value of the consumed current is exceeded. The FIU scheme is shown in fig. 2.
A clock pulse generator is made on the DD1 chip. Their frequency is regulated by a variable resistor R4.1 from 30 to 400 Hz. The pulse frequency at the outputs of the DD4 and DD5 microcircuits is six times lower - from 5 to 66,7 Hz. The current of just such a frequency will flow in the windings of the motor M1 (see Fig. 1), setting the frequency of rotation of its shaft. It is not worth reducing the frequency below the specified limit, the uneven rotation of the shaft will become noticeable. And at a frequency higher than the nominal one (50 Hz), the torque on the motor shaft drops sharply. Chains R5VD3C3-R10VD8C8 delay the fronts of the control pulses, leaving their recessions undelayed. This is necessary so that the output transistors of the keys that make up a pair (for example, SK1.1 and SK1.2), even for a very short time, do not turn out to be open at the same time, which would be equivalent to a short circuit of a 300 V DC voltage source and would lead, at best, to overheating, and at worst - to the failure of these transistors, and with them other elements of the SC. The inputs of the logic elements DD6.1-DD6.4, DD2.3, DD2.4, in addition to pulses with a frequency of 5 ... 66,7 Hz, receive higher-frequency pulses of adjustable duty cycle from the generator on the elements DD2.1, DD2.2. Variable resistors R4.1 and R4.2 are paired, therefore, at the outputs of the above elements, simultaneously with a change in the repetition frequency of the bursts, the duty cycle of the pulses filling these bursts changes. Resistors R2 and R3 are selected in such a way that at nominal or increased speeds, almost full voltage is supplied to the engine, and with their decrease, it decreases by about half. As a result, at a frequency reduced by a factor of ten, the current consumed by the electric motor only slightly exceeds the rated current. Inverters DD7.1-DD7.6 with increased load capacity serve as buffer elements. Their output circuits include LEDs of optocouplers installed in the SK1-SKZ switches and providing galvanic isolation between the control circuits and power units of the source. The SC scheme is shown in fig. 3. There are six such keys in total (two for each phase). At time intervals when no current flows through the optocoupler U1 LED, as a result of which its photodiode has a high resistance, transistors VT1 and VT2 are open, VT3 and VT4 are closed - the key is open. When current flows through the LED, the switch is closed. Elements VD3-VD6, R3 and C1 provide forced closing of the transistor VT4, which reduces energy losses and facilitates the thermal regime of the key.
Diode VD7 protects the transistor VT4 from voltage surges on an inductive load. You can learn more about the design of power keys and how to protect them in the book [4]. Before getting to know her, the author burned a lot of expensive high-power transistors. The BPZ scheme is shown in fig. four.
Four rectifiers are connected to the secondary windings of the transformer T1. The first of them, on the diode bridge VD1, serves to power the control units of the keys SK1.2-SKZ.2. From it, through the stabilizer on the transistor VT1, the microcircuits of the FIA are fed. Three isolated rectifiers on diode bridges VD1.1-VD3.1 serve to power the control units of switches SK2 - SK4, which are under high potential. The power rectifier is assembled on VD7-VD10 diodes and equipped with a C7L1C8 smoothing filter. By pressing the SB2 button, the winding circuit of the KM1 contactor is closed. The triggered contactor remains in this state due to the closed contacts KM1.2. Voltage 220 V, 50 Hz is supplied to the diode bridge VD7-VD10 through the closed contacts KM 1.1 and the primary winding of the current transformer T2. The contactor and motor M1 are turned off (see Fig. 1) by pressing the button SB1. The voltage on the secondary winding of the transformer T2, rectified by the diode bridge VD6, is proportional to the current consumed from the network. As soon as part of this voltage, removed from the engine of the variable resistor R2, exceeds the opening threshold of the trinistor VS1, the relay K1 will work and open the winding circuit of the contactor KM1.1 with its contacts K1, disconnecting the power rectifier from the network. Transformer T1 with an overall power of at least 60 W must have four well-insulated secondary windings for a voltage of 12 V. Winding II - for a current of 2 A. Windings III-V - for 0,7 A. Instead of a multi-winding, you can use several transformers with fewer windings. The magnetic circuit of the transformer T2 is a K28x6x9 ring made of ferrite 2000NM. Its secondary winding contains 300 turns of PEL 0,22 wire, and the role of the primary is played by the wire passed into the hole of the ring, going to the diode bridge VD7-VD10. Relay K1 - RES22 (RF4.500.121) can be replaced by any relay with a response voltage of 12 V and at least one group of normally closed contacts. The KM1 contactor with a 220 V winding is selected based on the power of the electric motor. Coils L1 and L2 (Fig. 1) are frameless, contain 25 turns of PEL 1,5 wire, wound in bulk on a mandrel with a diameter of 30 mm. The details and design of the SC units (see Fig. 3) should be treated with special attention. It is these nodes that bring the most trouble and material damage in the event of failure. Before installation, all parts must be carefully checked, and "suspicious" ones are mercilessly rejected. The VT4 transistor is installed on a heat sink of sufficient area (in the author's version - 400 cm2). A transistor VT3 is placed next to it on the same heat sink, and the leads of the VD7 diode are soldered directly to the leads of the transistor VT4. A pair of transistors KT8110A, KT8155A can be replaced with one composite MTKD-40-5-3. It is equipped with an internal protective diode, so the VD7 diode is not needed in the event of such a replacement. MTKD-40-5-2 composite transistors close in parameters are not suitable in this case, since they do not have an external output of the base of the second (powerful) transistor. The heat-removing surface of the MTKD-40 5 3 transistors is electrically isolated from the semiconductor structure, so the transistors of all switches can be installed on a common heat sink. All power circuits must be made with rigid, as short and straight wires as possible and removed from the FIS circuits. The cross section of each wire must correspond to the flowing current. Moreover, it is dangerous not only to underestimate, but also to overestimate the diameter of the wires. Circuits VD1L1 and VD2L2 (see Fig. 1) are mounted in the immediate vicinity of the keys, soldering them to the terminals of the corresponding transistors. If the block of power switches did not turn out to be compact, it is desirable to supply each pair of SCs with similar protective circuits. When setting up a source, first of all, using an oscilloscope, they check the presence and shape of pulses at the outputs of the FMU microcircuits. Then, without applying voltage to the VD7-VD10 diode bridge (see Fig. 4) and without connecting the M1 motor, they check whether the pulses are received at the bases transistors VT3 in all SCs. After that, the FIA is turned off, and the mains voltage is applied to the diode bridge through an adjustable autotransformer, gradually increasing it from 0 to 220 V. The engine remains unconnected. The consumed C K current should not exceed several tens of microamperes. After making sure of this, they lower the voltage at the output of the autotransformer to zero and, temporarily blocking the PWM (for this, it is enough to break the wire connecting the output of the DD2.2 element with the inputs of the elements DD2.3, DD2.4, DD5.1-DD5.4) , include PFI. Again, gradually increasing the voltage supplied to the SC, check the current consumed. It will become larger, but even at the maximum frequency it should not exceed 100 μA. The operation is repeated by unlocking the PWM and controlling the voltage shape at the points intended for connecting the motor windings with an oscilloscope. If all checks were successful, you can connect a three-phase electric motor of relatively small power (up to 1 kW) to the source and check its operation at reduced idle voltage, and then at rated mains voltage and mechanical load. The temperature of the power transistors and the total current drawn from the network should be constantly monitored. After making sure that the source is fully operational, it is possible to power electric motors with a power of up to 3,5 kW from it. Literature
Author: V.Naryzhny, Bataysk, Rostov region.
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